Metal additive manufacturing regularly generates headlines focused on in its capabilities in the production of new parts, but its applications in the modification or repair of existing components is often overlooked – not least in the field of welding. By Kenneth Vartanin and Pascal Pierra of Optomec.
Metal additive manufacturing (AM) technology is gaining momentum, but the level of adoption is still quite low compared with traditional subtractive metalworking methods. In 2016, only 957 metal AM machines were sold worldwide, compared with hundreds of thousands of CNC subtractive machine tools.
Part of the reason for the low adoption level may be the misconception that metal AM technology can only be used to 3D print new parts, which limits its potential use. This may be due to the fact that the most commonly used metal AM technology, powder bed fusion (PBF), can only 3D print new parts built-up on flat two-dimensional plates. However, metal AM technologies, such as powder-fed directed energy deposition (DED), can not only 3D print new metal parts, but can also add materials to existing components, enabling a broader range of applications such as repair, surface modification, and hybrid manufacturing (which combines traditional processes together with AM).
In essence DED is an automated precision welding process for building or repairing parts. With the global shortage of skilled welders, industry must find new methods to make their current workforce more productive. Could welding be the Trojan horse to accelerate industrial adoption of metal AM? In this article, we present new DED applications that can make it easier to justify and deploy metal AM applications.
Technological background
Laser-based AM technology was invented in the 1980s, and the two most commonly known methods available for commercial use today are PBF and powder fed DED systems. PBF systems use a laser to selectively melt a bed of metallic powder layer by layer to build up the physical part. After the first layer is spread and sintered, the bed is filled again with a second layer of powder and selectively sintered. This process is repeated until the part is fully formed. The end result is buried in a cake of powder and is not visible until the excess powder is removed.
Powder-fed DED systems continuously blow powder through nozzles directed at the focal point of a high-powered laser. The resultant molten pool of metal (sometimes referred to as a weld pool) is then moved using a motion control system and the part is built up in free space. The entire process is visible as the part is grown layer by layer.
Each process has its advantages. The PBF method is better at building smaller, more complex-shaped parts, and produces a better surface finish. The powder fed DED method is faster and better at adding material to existing parts (as in repair or hybrid manufacturing). In general, powder-fed DED technology produces fully-dense material that has excellent mechanical and fatigue properties. For building small metal components, PBF machines can be used in many cases, except if a functionally graded material is required, or if the desired material is one not commonly processed by PBF systems. For building large parts or repairing worn or defective metal components, powder-fed DED machines, such as the LENS machines offered by Optomec, are stronger candidates.
LENS DED – An automated precision welding process
The size of the weld bead produced with the LENS DED process can be as small 200 microns or as large as 6mm in diameter. This flexibility enables a range of applications from building thin wall structures to precision repair to adding wear resistant coatings to components. Unlike MIG or TIG welding, the highly targeted nature of the LENS DED process enables addition of materials to metal components with minimal heat affect to the surrounding area.
For companies that work exclusively with materials such as stainless and tool steels, Inconel, and other non-reactive metals, open atmosphere LENS systems are available. Just like in welding, an argon shield gas is used to protect the weld pool from contamination. For processing titanium, aluminium, refractories, and other reactive materials, LENS systems can be equipped with an atmosphere controlled hermetically sealed chamber that maintains oxygen and moisture levels below ten parts per million.
The LENS DED process also includes closed loop feedback controls that automatically adjust laser power to maintain a constant heat input to the melt pool and cooling rate during the deposition process. A constant cooling rate is essential to quality metal deposition. Because of these process attributes and controls, materials produced with the LENS process have excellent mechanical properties equal to or better than cast and in many cases similar to forged materials.
According to the American Welding Society (AWS), the US fabrication industry is now on course to have a shortage of 290,000 skilled welders by 2020. And this skilled workforce shortfall is not just a US problem. The average age of welders in Australia and the UK has now surpassed 55. The age profile of skilled welders has been skewed to the extent that shortages of welders is now an inevitable global problem. Experts in the field believe industry can’t train new welders fast enough to make-up for the shortfall.
So why is a shortage of skilled welders an important factor in driving adoption of DED AM technology? In an environment where there are not enough welding craftsmen, the only answer is to augment the skills of current and apprentice workforce with automated technology that make them more productive.
LENS DED welding applications
General Electric’s use of AM to print fuel nozzles for their LEAP engine is well known. The investment in engineering, process development, and equipment to achieve these results would be beyond the reach of most smaller companies. In the long run, General Electric’s investment will provide an ROI through savings across the product lifecycle. However, less well known are equally compelling applications where AM is used to repair components.
For example, the US Army Anniston depot utilised LENS technology to repair engine components for the Abrams M1 tank. Operating in a desert environment, M1 tank engines such as the Honeywell AGT1500 were experiencing extreme amounts of wear, requiring shorter interval maintenance cycles. The AGT1500 engine components are difficult to repair with traditional methods due to distortion effects caused by the high-heat welding process. With LENS, a highly focused laser beam delivered energy exactly to the repair area, reducing the heat affected zone (HAZ) enabling repair of these engine components. The LENS process allowed Anniston to repair worn engine components instead of replacing them, saving more than $5m per year.
Another example is an electronic satellite housing where a hybrid process was used to reduce manufacturing costs and time to market. In this example, the disk base of the housing was machined from a billet and the thin wall structures were built up using LENS DED. The hybrid process reduced lead time by six months while also eliminating the need for specialized tooling.
Even small organisations can benefit from LENS repair capabilities, such as a food factory based in Albuquerque in the US. Last December, 505 Southwestern’s salsa production line was shut down due to a broken helical gear. With no spare gears in stock and an eight-week lead time to deliver a replacement gear, 505 faced a significant loss in production capacity during the busy holiday season. Using LENS, the Inconel helical gear was repaired and the 505-production line was back in operation in one day.
These examples may be less exciting than printing a unitised jet engine fuel nozzle, but these applications are easier to deploy, provide a faster ROI, and also provide a pathway for organisations to develop more advanced AM applications for new product designs.
Breaking through the barriers
There have been many lessons learned in Optomec’s 20-year journey to commercialisation of AM technology. One of the biggest is that any new technology, especially a disruptive manufacturing technology such as AM, takes time to gain acceptance. This acceptance must be earned by finding cost effective applications that provide clear advantage over existing methods.
To enable a more compelling economic justification, LENS technology recently was integrated into lower-cost CNC platforms, making LENS metal AM much more affordable – as well as more familiar and intuitive to shopfloor operators. With entry level pricing below $250,000, LENS DED systems offer more capability at less than half the cost of competitive 3D metal printers and are even less expensive than many automated welding systems.
The risks are high for a manufacturer to implement any disruptive technology, so methods to mitigate risk can help accelerate deployment. Finding applications such as repair to extend product life or improve production yield are great examples. And the funny thing is, if you’re working with metal parts today, your organisation already utilises crude AM technology in your welding department. LENS DED technology enables your organisation to cost effectively advance your capability with precision welding while also providing a pathway to full 3D printing of new product designs.